Allocation of nutritional resources to reproduction (reproductive effort) in animals is subject to direct control by the environment as well as by the life history (litter size, developmental rate) of the species. There is considerable interest in the questions of evolution of life histories and their implications in terms of resource requirements. The purpose of this research is to compare cotton rats (Sigmodon hispidus) taken from a large litter size population (northern edge of species range) and a small litter size population (central part of range) in terms of their allocation of resources (energy, nitrogen, calcium, and phosphorus) to reproduction and parental maintenance. Experiments with restricted food levels will provide comparative data on the degree to which rats from the two populations will go into negative resource balance or sacrifice number versus size of individual offspring. Breeding experiments will verify that these differences are attributes of the rats from the two populations rather than direct environmental effects.